A leuco chromogen is a chromogen that generates a dye through reaction with hydrogen peroxide in the presence of a peroxidative substance such as peroxidase. Unlike a coupling-type chromogen, the leuco chromogen generates a dye by itself. For example, phenothiazine leuco chromogens, triphenylmethane leuco chromogens, and diphenylamine leuco chromogens are known (see e.g., patent documents 1 to 3).
The leuco chromogen is often used, as in the coupling-type chromogen, in the quantification of an analyte component such as cholesterol and glycated hemoglobin contained in a sample such as serum. Specifically, clinical laboratory examinations often involve: converting an analyte component in a sample into hydrogen peroxide; reacting the generated hydrogen peroxide with a leuco chromogen in the presence of a peroxidative substance such as peroxidase to convert the chromogen to a dye; and quantifying the analyte component in the sample on the basis of the absorbance of the generated dye. Particularly, the leuco chromogen is preferably used as a highly sensitive chromogen in the quantification of an analyte component contained only in a trace amount in a sample (see e.g., non-patent document 1).
Thus, the leuco chromogen is used as a highly sensitive chromogen in the quantification of a trace amount of an analyte component in a sample, while the leuco chromogen has poor storage stability and undesirably develops color spontaneously with time, particularly in a solution. To solve this problem of poor stability of the leuco chromogen, methods for stabilizing the leuco chromogen in a solution have been studied and reported so far (see e.g., patent documents 4 and 5). These methods for stabilizing the leuco chromogen, however, are not always satisfactory, for example, because they must be performed under strict conditions.